Safety belt pretensioners remove slack from a safety belt in the event of a collision in order to minimize forward movement of the passenger. It is known to use pyrotechnic gas generators to operate pretensioner mechanisms that wind up or otherwise pull in slack in the safety belt during a collision. Such pyrotechnic gas generators are often disposed internally of a safety belt retractor. High-temperature gases tend to abrade interior metal surfaces and produce ash and clinkers that bind up the retraction mechanism. Thus, the vehicle owner is faced with a significant cost penalty in that the entire pretensioner and retractor assembly must be replaced after activation because of the inability to prevent degradation of the retractor.
Further, where pyrotechnic gas generators are used to supply the rapidly increasing gas pressure for the pretensioner, the gas pressurization rate and the resultant initial driving force or acceleration of the piston can exceed the structural capability of the driven components. As a result, the entire pretensioner assembly can malfunction due to fracturing of a given part. If damaged due to application of excessive forces, the retractor will not retract and function as a load limiter during a second impact. Thus, the retractor must be replaced.
In addition, many existing retractor designs incorporate the pretensioner into the retractor assembly. In these designs, the retractor tends to occupy valuable space in the retractor assembly that could be used for other useful mechanisms, such as load-limiting devices.